Severe malarial anemia (SMA) is the most common clinical manifestation of severe malaria in infants and young children living in holoendemic Plasmodium falciparum transmission areas. SMA accounts for the greatest degree of global malaria-attributable morbidity and mortality. The pathophysiology of SMA is complex and multi-factorial and has been attributed to direct and indirect destruction of infected and uninfected erythrocytes, suppression of erythropoiesis, erythrophagocytosis, and dysregulation in cytokine production. However, the paucity of information on the immunological basis of SMA is a significant hindrance since a successful malaria vaccine must protect against development of life-threatening SMA. Different B cell subsets regulate protective and pathogenic immune responses associated with disease outcomes in a number of inflammatory mediated-diseases. Previous studies showed that alterations in the circulating memory B cells and regulatory B cells are associated with disease outcomes in rheumatoid arthritis (RA). Additional studies demonstrated that decreased surface expression of CXCR4 and CXCR5 and increased intracellular SDF-1&#945; and BCA-1 expression are associated with chronic human immunodeficiency virus, systemic lupus erythromatosus, and RA. Since nave, memory and effector B cell subsets predominantly produce IL-10, TNF-&#945;, and IFN-&#947;, differential expression of these subsets may be important determinants of disease severity in children with SMA. Studies in human malaria indicate that B cells mediate malarial immunity through gradual accumulation of specific memory B cells and antibody titers following repeated infection. Since cytokines/chemokines, and their receptors, can modulate B cell subsets and erythropoiesis, the current proposal will explore the role of different B cell subsets (expressing CXCR4/BCA-1 and CXCR5/SDF-1&#945;) in conditioning the development of SMA and suppression of erythroid development. Additional studies showed that increased activation of B cells through the IgE/CD23 pathway promotes increased production of erythropoietic suppressive mediators such as MIP-1&#945;. In addition, elevated IgE and soluble (s)CD23 are associated with increased severity of pediatric malaria. Moreover, increased activation of the IgE/CD23 axis is associated with profound anemia in murine models. Since, both CD23 and MIP-1&#945; regulate IgE synthesis, and are inversely correlated with hemoglobin levels, we will determine the effect of B cell expression of IgE/CD23 and MIP-1&#945; on the development of SMA. This proposal will also determine the effect of Plasmodium falciparum-derived hemozoin (PfHz) on IgE class switching and cytokine/chemokine B cell expression following activation of TLR9 (CD289). Although PfHz can promote antibody class switching and activation of CD289, no studies to date have been performed in children with SMA. The overall objective of this proposal is to investigate the role of different B cell subsets and the IgE/CD23 axis in regulatin the development of SMA in children less than 3 years of age residing in a holoendemic P. falciparum transmission area of western Kenya. The overall hypothesis of this proposal is that PfHz alters the production of cytokines (IL-10, TNF-&#945;, and IFN-&#947;) and chemokines (SDF-1&#945;, BCA-1 and MIP-1&#945;) which, in turn, promote differentiation of B cell subsets into different effector and regulatory phenotypes. To accomplish the experimental objectives, we will utilize a hospital-based prospective study with a longitudinal follow-up perio of 36 months. Since the underlying molecular basis required for development of protective immunity against SMA remains largely undefined, our investigations will focus on the ability of cytokines (IL-10, TNF-&#945;, and IFN-&#947;), chemokines (i.e., CXCR4/SDF-1&#945;, CXCR5/BCA-1 and CCR5/MIP-1&#945;), and PfHz to modulate B cell subsets and development of SMA. Since this proposal will perform investigations in extensively phenotyped cohorts of children, we will maximize our ability to successfully identify those B cell subsets tha influence the development of SMA.